OBJECTIVES: (1) Conclusive evidence on the age-dependent decrease in the reduced cysteine (SH) of H3 histone of rat liver and brain will be obtained. The decrease in the SH group of H3 histone appears to be due to blockage of SH of cysteine and this could perturb the structure and function of chromatin. (2) DNA-binding protease (25,000 dalton) is present in thymus chromatin. The protease appears to be present in the medullary small lymphocytes in thymus, and the level of the enzyme increases as much as 10 fold in 18 month old rat compared to 3 month old rat. We are presently testing the possibility of the protease being involved in the death of thymus cells.